Diet is highly individualised and specific to people
That being said, you mentioned healthiness.
Health is not measured strictly by diet.
There could be alternate factors those eating a specific diet could have that impacts on their health.
E.g., Those following a Mediterranean diet may have a more supportive social culture, or those on a Vegan diet supporting each other for animal rights.
Also I doubt the meat in Europe and America is different enough to make a large difference in nutrition. It’s more likely due to the difference of overall food in Europe and America.
The Mediterranean diet is generally regarding as the healthiest diet. Predominantly fish, some chicken, and the odd bit of red meat (none processed) is ideal. The vast majority of your diet should be fruit and vegetables with whole grain foods. I’d guess you just didn’t have a balanced diet while you were heavily meat based, the ideal diet is actually quite similar to vegetarian but just small amounts of meat (sheer protein content, the amino acid profile and bioavailability is unmatched compared to vegetarian products). Balance and variety is key!
I’m mostly plants for a few years. Cool with eggs, fermented dairy, and occasional bit of salmon. But zero mammal meats.
Definitely felt better with more plants but also stopped processed foods in general. Who’s to say?
You can eat shit on a vegan diet (Oreo diet). The healthiest version of vegan diet is one that changes the way you look at a meal. Like if you go vegan and try to emulate chicken and rice you’ll probably end up worsening your food quality.
The hardest thing imo is that we tend to look for the protein replacement, then we look at protein per 100g of produce and we find it almost impossible to compete with animal foods. Animal foods are much more nutrient dense, it’s true, veggies contain mainly water. So start looking at protein as a % of calories and you’ll realize it’s super easy to match animal products?